Virtual Threads (Java 21)

Threads that are cheap

A virtual thread is scheduled by the JVM, not the OS, so you can run millions of them. When one blocks on I/O, the JVM parks it and reuses the carrier thread. This keeps the simple thread-per-request style scalable.

Creating virtual threads

Use Thread.ofVirtual() or an executor that starts one virtual thread per task.

Thread.ofVirtual().name("worker-1").start(() -> fetch(url));

try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
    for (String u : urls) {
        executor.submit(() -> fetch(u));   // one cheap thread per task
    }
}   // close() waits for all tasks

Virtual threads help I/O-bound work, not CPU-bound loops. Do not pool them, avoid long synchronized blocks around blocking calls, and use ReentrantLock instead.

Quick check

Quick check: What are virtual threads best suited for?

• Many concurrent blocking I/O tasks
• CPU-bound number crunching only
• Replacing the garbage collector
• Compiling code